In a photoconductor drum substrate for use in electrophotographic devices such as, copiers, laser beam printers, facsimile devices, etc., a photosensitive layer such as an OPC layer is thinly applied on the outer surface so that the thickness becomes uniform.
An outer surface of an aluminum alloy tube used for the substrate is required to have a high surface smoothness so that a photosensitive layer having a uniform thickness can be applied thereon.
Conventionally, a high surface smoothness was obtained by machining an outer surface of an aluminum alloy tube. In the method, however, since it is not easy to adjustment and/or manage the machining tools and the machining operation demand skill and experiences, there is a problem that this method is not suitable for mass production. A tube obtained by machining the outer surface as mentioned above is called a “machined tube.”
Under the circumstance, in recent years, a non-machined tube, such as a drawn tube obtained by drawing an aluminum alloy extruded tube, an ironed tube obtained by ironing an aluminum alloy extruded tube, etc., has come into use.
In such a non-machined tube, the surface quality of the outer surface is largely affected not only by the processing accuracy of the drawing process or the ironing process as a final process but also by the surface quality of the outer surface of the extruded tube. Therefore, in order to assuredly make the outer surface of the non-machined tube into a high smooth surface, it is required to improve the surface quality of the outer surface of the extruded tube.
As a technology to improve surface quality of an outer surface of an extruded tube, it is known, as described in Japanese Unexamined Patent Application Publication H7-284840 (JP-7-284840, A) (Patent Document 1), to extrude an aluminum billet using an extrusion die having a bearing portion made of WC-Co series superhard alloy containing cobalt of less than 16%, or as described in Japanese Unexamined Patent Application Publication No. 2004-358555 (JP-2004-358555, A) (Patent Document 2), to extrude an aluminum billet in which the maximum thickness of the solidified shell layer of the outer peripheral surface is 13 mm or less.